Data storage technologies have been dominated by magnetic and conventional optical data storage technologies, where data are stored as distinct magnetic or optical packages on the surface of a recording medium. These methods rely on the surface area available and do not utilize the entire volume of the recording medium. Holographic data storage is a promising approach of utilizing the entire volume of the recording medium for data storage.
In holographic data storage, data is stored as an optical interference pattern within a volume of recording medium of photosensitive optical material such as a photorefractive crystal. This is done by intersecting two laser beams within the recording medium. The first laser beam is the object beam and contains the data to be stored. The second laser beam is referred to as the reference beam and is used for reproducing the stored data from the recording medium. The resulting optical interference pattern causes chemical and/or physical changes in the recording medium. A replica of the interference pattern is stored as a change in the refractive index of the recording medium.
A large number of these interference patterns can be superimposed in the same piece of recording medium as long as they are distinguishable by the spacing of the gratings. Such spacing can be created by varying angles between the object and reference beams, which is referred to as Angular Multiplexing. Alternatively, varying the wavelengths of the lasers in a technique known as Wavelength Multiplexing can achieve a similar result. Other techniques such as Phase Code, Peristropic and Shift Multiplexing are also implemented. Using such multiplexing techniques, the theoretical limit of storage density in holographic data storage is around tens of terabits per cubic centimeter.
In addition to high storage density, holographic data storage allows for very high data transfer rates and fast access times, as data is recorded page by page and laser beams can be manipulated without inertia, unlike actuators and motors in disk drives.
Despite the advantages of holographic data storage, there are limitations caused by the quality and the effectiveness of the recording medium. The recording medium is primarily a crystal and the main workhorses in the field of holographic recording are Lithium Niobate (LiNbO3) crystals.
Lithium Niobate has favorable photorefractive characteristics that make it suitable for holographic recording. Doping Lithium Niobate with transition-metal or rare-earth ions such as Iron (Fe) and Cerium (Ce) enhances these characteristics. At present iron doped Lithium Niobate (Fe:LiNbO3) is considered one of the best options for a recording medium. Using stoichiometric LiNbO3 ([Li]/[Nb]=1) has further enhanced the performance of the crystal especially when doped with Fe.
However, regardless of the desirability of using Fe doped LiNbO3 crystal, it inherently has some bottlenecks that limit the operating ranges of the crystal. The relatively low sensitivity of Fe:LiNbO3 has limited the data recording speed. Strong beam-fanning effects and low optical damage thresholds prevent the use of focused laser beams with high power density in increasing the data recording speed and density on the holographic medium.
It can thus be seen that there exists a need for improved doped Stoichiometric Lithium Niobate (SLN) crystals for high-speed holographic recordings that can overcome the disadvantages of the existing art.